1. Field of the Invention
The present invention relates to an enhancement/depletion (ED) inverter circuit including a transistor made of a nitride-based compound semiconductor and an integrated circuit element including the ED inverter circuit.
2. Description of the Related Art
The wide bandgap semiconductors represented by III-V nitride-based compound semiconductors are appropriate materials for high-temperature, high-power, and high-frequency semiconductor devices due to their electrical characteristics including high breakdown voltage, preferable electronic transport property, and preferable thermal conductivity. For example, in a field-effect transistor (FET) having an AlGaN/GaN heterostructure, a two-dimensional electron gas is generated on a boundary of the heterostructure due to a piezoelectric effect. High electron mobility and high carrier concentration of the electron gas attract a considerable attention. Moreover, because a heterojunction field-effect transistor (HFET) having the AlGaN/GaN heterostructure has a wide band gap, low on-resistance and high switching speed, the HFET is operable in a high-temperature environment. Therefore, it is expected that an integrated circuit (IC), such as an inverter circuit, based on the nitride-based compound semiconductor can be used in a higher-temperature environment in which a conventional IC can hardly be used.
However, because of a difficulty in forming a p-type semiconductor using an ion implantation in a nitride-based semiconductor transistor, it is difficult to fabricate a complementary metal-oxide semiconductor (CMOS) device. As a result, it is necessary to use an enhancement/depletion (ED) inverter circuit as a basic unit. An ED inverter circuit formed with an enhancement-type high electron mobility transistor (HEMT) and a depletion-type HEMT is disclosed in, for example, “Recessed-Gate Structure Approach Toward Normally Off High-Voltage AlGaN/GaN HEMT for Power Electronics Application”, Wataru Saito, et al., IEEE Transactions on Electron Devices. vol. 53, No. 2, pp. 356-362, 2006 (hereinafter, “a first literature”) and “Monolithically Integrated Enhancement/Depletion-Mode AlGaN/GaN HEMT Inverters and Ring Oscillators Using CF4 Plasma Treatment”, Yong Cai, et al., IEEE Transactions on Electron Devices. vol. 53, No. 9, pp. 2223-2230, 2006 (hereinafter, “a second literature”). However, the HEMTs are basically the depletion-type HEMTs. In the first literature, the enhancement-type HEMT is formed by recess-etching an AlGaN layer that is formed under a gate electrode. In the second literature, the enhancement-type HEMT is formed by doping fluorine as an n-type dopant in the AlGaN layer that is formed under the gate electrode.
However, in the HEMT disclosed in the first literature, because a thickness of the AlGaN layer is decreased due to the recess-etching of the AlGaN layer, the electrical characteristics of the electron gas is degraded. Moreover, because the AlGaN layer having hard crystal structure is thin, it is difficult to obtain a desired recess depth by the recess-etching. Therefore, it is difficult to obtain a desired threshold voltage because the threshold voltage of the HEMT depends on the recess depth. In the HEMT disclosed in the second literature, the fluorine is doped in the AlGaN layer by using a plasma treatment in a relatively low temperature environment. Accordingly, a fluorine diffusion is likely to happen particularly when the HEMT is used at a high temperature, which can degrade reliability of the HEMT.